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Appendix IV.                            965

instantaneously in one flash. In other engines, like the original
Benz, having but 800 revs, per m. or thereabouts, a trembling
hammer on the coil causes a spark of longer duration.

The magneto-machine is represented by the Sims-Bosch gear*
shewn in Fig. 899. A is a permanent magnet of some strength,
and B a fixed armature wound lengthwise with a wire c making
circuit with the sparking plug E partly directly and partly through
the engine frame D. F is a soft iron shield which is caused to
rock on the armature centre by means of the connecting rod F
coupled to a cam disc G on the engine shaft. As the cam rotates,
it allows the tappet H to lower at the proper time and raise the
trip lever j, thus breaking the armature circuit and causing the
spark to pass at E within the cylinder. It is well known that the
disturbance of a magnet's field will cause difference of potential
in a circuit between its poles, and such disturbance is actually
produced by the rocking of the soft iron shield.

Fig. 900 shews the method of connecting up a small shunt-
wound dynamo A, which is driven from the engine shaft. As the
current cannot flow till the engine is running, the latter must be
started by means of the secondary battery B, which is provided
with a primary coil c having large self-induction; but as soon as
the dynamo becomes excited by the engine it is switched in
circuit with the coil and firing plug, while the accumulators are
fed in parallel by a portion of the dynamo current, so that the
cells are kept constantly charged. Thus by closing switch E, we
connect the battery B, coil c, and firing plug; and when ready the
dynamo is coupled by switch D, sending some of its current by
c and F, and the overflow through B. When the cells are fully
charged, E may be opened.